The Double-Slit Experiment
Throw marbles at two slits and you get two stripes. Send ripples and you get many. So what do electrons, which are tiny particles, do? The answer is the strangest result in physics.
Throw marbles
A wall with two big slits, and a screen behind it. Each marble goes through one slit or the other.
Two slits, two stripes, one behind each slit. That's what particles do.
Now make waves
Ripples in a pond. A ripple goes through both slits at once, and the two new sets of ripples cross.
Many stripes, not two. That's what waves do.
- Sunlight through a pinhole in a window shutter
- Split in two by the edge of a thin card
- Stripes on the wall
Same stripes as the ripples. Proof that light is a wave.
Now electrons: tiny particles
An electron is a particle, like a very tiny marble: it hits the screen as one dot. Fire them one at a time.
Guess first: what will they make?
Spy on the slits
Add a detector that flashes at whichever slit each electron goes through. Now we'll know which one it took.
Look, and the stripes vanish. Caught at one slit, it stops acting like a wave: just a plain smudge of dots, no stripes.
What it means
A marble that moves like a ripple
Electrons, light, even whole molecules: each one travels like a wave, through both slits at once, and lands like a marble, in one spot. The wave sets the odds; exactly where each one lands is pure chance. Check which slit it used, and it goes back to being a plain marble.
“A phenomenon which is impossible, absolutely impossible, to explain in any classical way, and which has in it the heart of quantum mechanics.” Richard Feynman
The stripes come from the standard two-slit formula, worked out in your browser; each dot lands at a spot drawn at random from it. Sizes and speeds are scaled up to be visible.
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